Clipboard in hand, Dr Francois Nosten worked his way down a ward of malaria patients. He stopped in front of five-year-old Ayemyint Than, who sat to attention and smiled. The smile told Nosten as much as his lines of graphs and figures.

Ayemyint Than, 5, is being treated for malaria in northwestern Thailand.

“She’s doing well,” he said, moving to an older man, whose pale face and dull sunken eyes told a very different story. “Day five, and he’s still positive?” he asked another of the doctors. “That’s not very good. It means he was very slow to clear the parasite, no?”

To Nosten, it was further evidence of an alarming rise in resistance to artemisinin, currently the front-line drug in the treatment of malaria. He fears it could be the start of a global “nightmare” in which millions of people could lose their lives.